The present invention relates generally to vapor compression refrigeration systems, and more particularly to a vapor compression refrigeration system having a variable capacity, multi-circuit refrigerant condenser.
Compression refrigeration systems generally comprise a compressor, a condenser, an expansion device, and an evaporator connected by appropriate refrigerant lines to form a refrigeration circuit. Refrigerant vapor is compressed by the compressor and fed to the condenser where the refrigerant releases heat to a cooling medium and condenses. The condensed refrigerant then flows through the expansion device where the pressure of the refrigerant is reduced. From the expansion device, the refrigerant passes into the evaporator, absorbs ambient heat, and vaporizes. Vaporous refrigerant is then drawn back into the compressor, completing the circuit.
Refrigeration systems of the foregoing type are frequently used in situations such as commercial and industrial buildings where the load upon the system may vary. It is common to design the system to meet the maximum load to which it may be subjected, and to reduce system capacity under low load conditions. Frequently, such a system may have a plurality of compressors wherein system capacity is reduced by removing one or more compressors from operation, or the system may have a variable capacity compressor wherein system capacity is reduced by reducing compressor capacity, for example by partially or completely unloading some of the piston cylinders of the compressor if the compressor is of the reciprocating piston type.
Variable capacity refrigeration systems generally include a condenser having a plurality of refrigerant circuits, and a plurality of high pressure fluid lines for conveying refrigerant from the compressor or compressors of the system to the condenser circuits thereof. Each condenser circuit includes a set of parallel tubes which commonly extend through a plurality of generally parallel, planar heat transfer fins. The tubes are generally perpendicular to the planes of the fins, with tubes at each end of the condenser being connected by return bends to form the circuit. These circuits are divided into a number of groups. Each group is supplied with refrigerant from a different one of the high pressure lines connecting the condenser with the compressor, usually via a refrigerant distribution device located in the line. One or more of the high pressure refrigerant lines may each include a valve which can be closed to prevent refrigerant from passing through the line. When refrigerant is prevented from passing through a high pressure refrigerant line, refrigerant is also prevented from passing through the condenser circuits which are fed by that line. Alternately, each line can be supplied with refrigerant by a separate compressor, in which case refrigerant can be prevented from passing through selected condenser circuits by shutting down the compressor supplying the line which feeds the circuits.
Under low load conditions, the capacity of the refrigeration system is reduced, and refrigerant flow through one or more high pressure refrigerant lines is prevented so that the refrigerant circuits fed by the closed line or lines are removed from operation. By removing selected groups of condenser circuits from operation in response to reduction in compressor capacity, a relatively high vapor pressure can be more easily maintained in those circuits which remain in operation. This improves condenser performance. For example, preserving a high vapor pressure facilitates maintaining stable vapor flow through the heat exchange tubes of the condenser. Further, since vapor temperature generally increases with pressure, maintaining a high vapor pressure preserves a high vapor temperature; and this maintains a larger temperature difference between the refrigerant vapor and the cooling medium, improving heat transfer therebetween. With prior art variable capacity condensers, however, a concomitant result of removing selected groups of condenser refrigerant circuits from operation has been to reduce the effective heat exchange area of the condenser. This detrimentally affects condenser performance.